1. Field of the Invention
The present invention relates to a zoom lens device and an image pickup apparatus including the same. The zoom lens device and the image pickup apparatus are suitable for use in, for example, a video camera, an electronic still camera, a broadcasting camera, or a silver halide photographic camera.
2. Description of the Related Art
In recent years, an image pickup apparatus, such as a camera using a silver-halide film, a broadcasting camera, a digital still camera, or a video camera using a solid-state image pickup element, is becoming a highly functional apparatus, and the entire image pickup apparatus is being reduced in size. Accordingly, a zoom lens device, serving as an image taking optical system and used in the image pickup apparatus, is required to have a short overall length, to be compact, and to have a high resolution.
Further, the zoom lens device is required to have a wide field angle and a high zoom ratio.
A known zoom lens device meeting such requirements includes, from an object side to an image side, four lenses in the following order. They are a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power.
A zoom lens device including five lens units is also known. The five lens units are, from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, a fourth lens unit having a positive refractive power, and a fifth lens unit having a positive or a negative refractive power.
A rear-focusing type zoom lens device including four lens units is known. In the zoom lens device, a first lens unit, a second lens unit, and a third lens unit perform zooming, and a fourth lens unit corrects an image plane variation, resulting from the zooming, and performs focusing. (Refer to U.S. Pat. No. 7,057,818, U.S. Pat. No. 7,193,790, and Japanese Patent Laid-Open Nos. 08-050244, 2004-199000, and 2006-113453.)
A zoom lens device including five lens units and whose fifth lens unit has a positive refractive power is known. In the zoom lens device, during zooming, first to fourth lens units are moved to set a high zoom ratio. (Refer to Japanese Patent Laid-Open No. 2007-003554.)
In general, to reduce the size of the entire zoom lens device, the number of lenses is reduced while increasing the refractive power of each lens unit of the zoom lens device.
However, in such a zoom lens device, a lens wall thickness is increased due to an increase in the refractive power of each surface. When a contraction effect of a lens system is not sufficient, it becomes difficult to correct various aberrations at the same time.
In the aforementioned zoom lens device including four lens units and zoom lens device including five lens units, for obtaining good optical performance while reducing the size of an entire lens system and achieving a high zoom ratio, it is important to properly set, for example, the refractive power of each lens unit, a lens structure, and movement conditions resulting from zooming of each lens unit.
In particular, for reducing an effective diameter of the first lens unit (front lens unit) while achieving a high zoom ratio, it becomes important to move the first lens unit during zooming, and to properly set the movement conditions of the first lens unit resulting from the zooming at this time.
In addition, it becomes important to properly set the refractive powers of the first and second lens units and an imaging magnification resulting from zooming.
When these structural factors are not properly set, it becomes very difficult to obtain a high optical performance within an entire zooming range while maintaining a high zoom lens device ratio.